A gas turbine includes a compressor section that produces compressed air that is subsequently heated by burning a fuel in the reaction zone of a combustion section. The hot gas from the combustion section is directed to a turbine section where the hot gas is used to drive a rotor shaft to produce power. The combustion section typically includes a casing that forms a chamber that receives compressor discharge air from the compressor section. A number of cylindrical combustors typically are disposed in the chamber and receive the compressor discharge air along with the fuel to be burned. A duct is connected to the aft end of each combustor and serves to direct the hot gas from the combustor to the turbine section.
Due to rising fuel costs, gas fired power plants that were designed to operate at mostly full power output are now being operated on a intermittent basis. Coal and nuclear energy generally may make up the majority of stable power output. Gas turbines increasingly are being used to make up the difference during peak demand periods. For example, a gas turbine may be used only during the daytime and then taken off line during the nighttime when the power demand is lower.
During load reductions, or “turndowns,” combustion systems must be capable of remaining in emissions compliance down to about fifty percent (50%) of fill rated load output, or “base load.” In order to maintain acceptable fuel-to-air ratios at the required turndown levels and to control the formation of oxides of nitrogen (“NOx”) and carbon monoxide (CO), considered atmospheric pollutants, it is sometimes desirable to cause a portion of the compressor discharge air from the compressor section to bypass the combustors.
Previous bypass systems have accomplished this by reinjecting the bypass flow as a dilution jet directly into the duct that directs the hot gas to the turbine. This approach may suffer from several drawbacks. Reinjecting the bypass flow as a single dilution jet can cause flame quenching and high levels of atmospheric pollutants in combustion systems. In addition, introducing combustor bypass air directly into the duct at one localized spot may create distortions in the temperature pattern and profile of the hot gas flowing into the turbine section. Moreover, the effect on pattern and profile generally cannot be tailored to meet downstream hardware thermal requirements.
There is a desire therefore to provide an apparatus for causing a portion of the compressor discharge air to bypass the combustor and enter the hot gas flow path downstream of the combustor. Such a bypass may reduce the concern for quenching and atmospheric pollutants, prevent distortions in the gas temperature profile, and allow tailoring of the pattern and profile factors to meet downstream hardware thermal requirements.